1. Field of the Invention
The invention relates generally to pharmaceutical coatings, and more particularly to a pharmaceutical tablet, capsule, bead or particulate coating in the form of an aqueous latex of water insoluble polymeric compositions, and to solid pharmaceutical dosages coated with such materials.
2. Discussion of the Prior Art
The use of various coatings to seal pharmaceutical (including vitamins and food suppliments) dosages has been known for some time. Many ends have been served by such coatings. For instance, tablet coatings provide a more attractive appearing dosage, protect the dosage from attack, such as by moisture during storage, provide a more acceptable feed, and often taste, to the user of the dosage, and, with selected compounding, provides either gastric and/or enteric dissolution at varying rates to determine the time and rate at which the dosage is released.
Early coatings include natural and synthetic polymers such as cellulose acetate phthalate and shellac to, as the primary purpose, delay release of the dosage thus providing enteric dosage. Sugar coating of the dosage is a classic example of an early pharmaceutical tablet coating affording gastric release, but has the drawback of being a very involved process requiring highly skilled operators. As a result of the various advantages and disadvantages of known coatings for pharmaceutical dosages, film coatings applied to the dosages from organosols, i.e., from solutions of plasticized polymers and organic solvents, have become the most utilized procedure. Film coatings from organosols provide rapid, simple, effective coating of the dosage and control over the dosage size, coating disintegration and dissolution rates, and generally results in an attractive and desirable dosage coating. Generally, the organic solvents curtailed bacterial and mold growth.
A number of disadvantages have also been attendant to the organosol coatings of pharmaceutical dosages. Often the vapors of the organic solvents are toxic or flammable thereby providing a hazard to the operators-though the solvents may be fully or substantially driven from the tablet coating providing no hazard to the user of the pharmaceutical dosage. Also, organic solvents present an environmental hazard and, accordingly, require recovery systems which are both expensive and complicated. As a result, in an effort to reduce the amount of solvents employed in film coating, fillers and extenders are often added to the film coating formulations. While this results in an increased solids loading of the coating dispersion, the additives and extenders often compromise the performance of the tablet coating in terms of mechanical properties and permeability. Except in high volume operations, the cost of recovery equipment for the organic solvents often proves prohibitive despite the high cost of the solvents recovered by such systems for recycling. Independent of the fundamental economics of such recovery systems, the demands of environmentally responsible manufacturing often mandate such solvent recovery systems with resulting adverse impacts on the economics of many film coating operations.
While the European pharmaceutical industry has employed two aqueous based synthetic polymer dispersions for the coating of pharmaceutical dosages, i.e., Eudragit E-30D, a 70:30 ratio of ethylacrylate-methyl methacrylate copolymers with a preferred molecular weight of about 800,000; and Eudragit L-30D, a 50:50 ratio of methacrylic acid-ethyl acrylate copolymers with a preferred molecular weight of 250,000, which are produced by emulsion polymerization, these compounds are subject to bacterial and mold growth while in aqueous form.
Water, of course, is an excellent solvent or vehicle for many practical purposes in that it is nonflammable and thus provides no explosion hazard, is low cost, is substantially free of toxic effects and innocuous to the environment. Unfortunately, the use of aqueous solutions of polymers for film coating to provide dosage coatings is quite limited due to the very low solids contents, i.e., on the order of 7% to 15% of known aqueous solutions. As the concentration of solids rises, the viscosity of the solution rapidly approaches an inoperable condition. Similarly, an increase in the molecular weight of the polymer in an aqueous solution rapidly causes an unworkably high viscosity. Thus relatively low molecular weight polymers and low concentrations are normally required. These characteristics result in a need for a plurality of layers of polymer to be built up in order to obtain a coating of adequate thickness for surface protection. Multiple coatings involve a long processing time which is complicated further by the slow evaporation of water and large amounts of water which must be removed from each coating. Further, many drugs are sensitive to water when exposed to high concentrations for extended periods of time and, accordingly, require initial sealing with water barriers, such as the shellac coating, prior to coating with the conventional aqueous film coating.
Another serious drawback with existing water-based coating systems is the tendency for bacteria and molds to multiply and grow in aqueous media, either solvent or vehicle. This tendency has been observed in the abovementioned Eudragit compositions. Thus care in storage and attention to shelflife are of critical importance in avoiding septic conditions in known aqueous based coating systems.
Still another problem with the low solids aqueous film coating techniques now known is the relatively poor film obtained. Few water soluble materials provide the desired film coatings properties. The more desirable film obtainable with the use of longer chains, higher molecular weight polymers cannot be employed in the conventional aqueous coating methods.